Chapter 28
External factors and Plant growth
Nastic Movement
• Nastic Movements- plants movement that
occur in response to a stimulus independent
of position of stimulus ( leaves light and
dark cycle)
• Phototropism- response to light- caused by
elongation- under the influence of Auxin
• Gravitropism- response to gravity
• Thigmotropism- response to touch
External Factors and Plant Growth
• Tropism- the growth response involving
bending, or curving, of a plant part toward
or away from an external stimulus
determines the direction of movement
Went’s experiment- chemical produced by
growing tips influences direction of growth
What role does the light play in the
phototropic response?
• Light decreases the auxin sensitivity of the cells
on the lighted side
• Light destroys auxin
• Light drives auxin to the shaded side
‘Light eliminates auxin activity’
Tropism video
Gravitropism- starch-statolith plastids hypothesis
Auxin and root gravitropism
Gravitism video
Thigmotropism is growth in
response to touch
• Tendrils of bur cucumber
• Enables shoots and roots to navigate, cling
and climb
• Thigmotropism in Opuntia acanthocarpa
flowers video
Thigmotorpism- tendrils of
bur cucumber, twisting
Caused by different growth
Rates on the inside and
Outside of tendril
Phototropism
• Common response to
light
Diurnal movements of Wood sorrel (Oxalis)
Diurnal movement- biological clock is the organism
keeping itself in time with some external stimulus?
Other types of Movement
• Thigmonastic (seismonastic) Movement are
results of mechanical stimulation
• Most night closure (nyctinastic movement)
are results from changes in the size of
perenchyma cells in the jointlike thickening
(pulvinus) structure at the base of each leaf.
Pulvini Mimosa pudica vascular tissue surounded
By a cortex which consists largely of thin-walled
Parenchyma cells
Mimosa pudica
Entire leaf drops in response
To movement, shock, thermal
Stimulation, touch
Results from changes in turgor
Pressure in pulvini
Touch reponse in the venus flytrap (Dionaea muscipula)
Thigmomorphogenesis- The inhibition of growth by touch Arabidopsis
thaliana both six weeks of age
Solar tracking Lupine (Lupinus arizonicus)
Sunflower solar tracking